Single crystals of the cubic phase of zirconia are commonly used as a substitute for diamond in jewelry. Like diamond, cubic zirconia has a cubic crystal structure and a high index of refraction. Discerning a good quality cubic zirconia gem from a diamond is difficult, and most jewelers will have a thermal conductivity tester to identify cubic zircona by its low thermal conductivity (diamond is a very good thermal conductor). This state of zirconia is commonly called "cubic zirconium" or "zircon" by jewelers.

Cubic Zirconia is a cubic form of zirconium oxide that is created in a laboratory, thus it is not a mineral.  However, it was found naturally occurring once at one site in the 1930's, but has yet to be discovered since then. The mineral with the same chemical composition as CZ, but in the monoclinic crystal system, is baddeleyite.  

Cubic zirconia has a hardness of 8.5 on Moh's Scale of Hardness and a white streak.  It has a specific gravity between 5.65 and 5.95, and a density between 5.5 and 5.9.  Cubic zirconia crystallizes in the isometric system.  The refractive index of CZ is between 2.088 and 2.176, which is very high.  Since CZ is transparent, it is often faceted.  It can be made in nearly any color and can be faceted into many cuts.